U.S. Pat. No. 6,580,600 (corresponding to JP-A-2002-243690) discloses a capacitive humidity sensor 100 shown in FIG. 8A. The sensor 100 includes a detecting portion 10 and a reference portion 20, and a capacitance of the detecting portion 10 varies in accordance with humidity.
An operation principle for the capacitive humidity sensor 100 will be described with reference to FIGS. 8A and 8B. FIG. 8A is a plan view showing the capacitive humidity sensor 100, and FIG. 8B is a graph showing a relationship between a relative humidity and capacitances Cv, Cr of the detecting portion 10 and the reference portion 20.
The detecting portion 10 includes a pair of electrodes 131a, 131b on a substrate 110, and the reference portion 20 includes a pair of electrodes 132a, 132b on the same substrate 110. The electrodes 131a, 131b, 132a, 132b are shaped into teeth of a comb. A moisture-sensing film 151 is arranged on the electrodes 131a, 131b of the detecting portion 10, and a relative permittivity (dielectric constant) of the film 151 varies in accordance with humidity. In contrast, any moisture-sensing film is not arranged on the electrodes 132a, 132b of the reference portion 20. Therefore, the capacitance Cr of the reference portion 20 is approximately constant, while the capacitance Cv of the detecting portion 10 varies in accordance with humidity, as shown in FIG. 8B.
As shown in FIG. 8A, the detecting portion 10 is connected to the reference portion 20 in series. When the capacitance Cv of the detecting portion 10 is compared with the capacitance Cr of the reference portion 20, a ratio of a voltage V12 to a voltage V23 is defined as a comparison value V12/V23. The voltage V12 for the detecting portion 10 can be expressed as Formula 1, and the voltage V23 for the reference portion 20 can be expressed as Formula 2. A voltage V0 represents a sum of the voltages V12, V23.V12=V0·Cr/(Cv+Cr)  (Formula 1)V23=V0·Cv/(Cv+Cr)  (Formula 2)
A circuit (not shown), e.g., capacitance-voltage converting circuit, is arranged on the substrate 110, and calculates a relative humidity based on the comparison value V12/V23. Thus, the relative humidity can be measured.
However, a temperature dependency may be generated in a sensor output, because the moisture-sensing film 151 has a temperature dependency. That is, a temperature dependency may be generated in a sensitivity of the sensor 100, because an amount of moisture absorbed or desorbed by the film 151 is varied by a temperature. The temperature dependency of the sensor output cannot be reduced, because any moisture-sensing film is not arranged on the reference portion 20.
If the sensor 100 is disposed in a high-temperature and high-humidity condition for a long time, the electrodes 132a, 132b may deteriorate. The capacitance Cr of the reference portion 20 may fluctuate due to the deterioration. In order to reduce the deterioration, the reference portion 20 may be protected by a gel portion. However, because the dimensions of the sensor 100 are minimized, the detecting portion 10 adjacent to the reference portion 20 may also be covered with the gel portion. In this case, response performance of the detecting portion 10 may be lowered. Further, because a process for arranging the gel portion is needed, a manufacturing cost may be increased.
In contrast, U.S. Patent Application Publication 2006/0096370 A1 (corresponding to JP-A-2006-133191) discloses a capacitive humidity sensor including a first sensor element corresponding to the detecting portion 10 and a second sensor element corresponding to the reference portion 20. Moisture-sensing films are formed on the first and second sensor elements, respectively. The first sensor element has a gradient of a capacitance variation to a humidity variation, which is different from that of the second sensor element.
Thereby, a temperature dependency in a sensor output can be reduced, because the moisture-sensing film is arranged on the second sensor element, similarly to the first sensor element. Further, the sensor can be stably used in a high-temperature and high-humidity condition for a long time, because the film protects electrodes of the second sensor element. Furthermore, because the gel portion is not needed, response performance of the first sensor element can be kept better, and the manufacturing cost can be kept low.
However, a pattern (shape) of the electrodes is different between the first sensor element and the second sensor element. For example, a clearance between the electrodes is different between the first sensor element and the second sensor element. Therefore, a difference of initial capacitances of the elements may be large relative to a capacitance variation corresponding to a humidity variation from 0% RH to 100% RH. The initial capacitance represents a capacitance at a predetermined humidity, e.g., 0% RH or 100% RH. The large difference of the initial capacitances of the elements may generate noises.